GN: Add documentation for gyp command.

tools/gn/variables.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/variables.h"
 
 namespace variables {
 
 // Built-in variables ----------------------------------------------------------
 
@@ skipping 386 matching lines @@
 const char kExternal_HelpShort[] =
     "external: [boolean] Declares a target as externally generated.";
 const char kExternal_Help[] =
     "external: Declares a target as externally generated.\n"
     "\n"
     "  External targets are treated like normal targets as far as dependent\n"
     "  targets are concerned, but do not actually have their .ninja file\n"
     "  written to disk. This allows them to be generated by an external\n"
     "  program (e.g. GYP).\n"
     "\n"
+    "  See also \"gn help gyp\".\n"
+    "\n"
     "Example:\n"
     "  static_library(\"foo\") {\n"
     "    external = true\n"
     "  }\n";
 
 const char kForwardDependentConfigsFrom[] = "forward_dependent_configs_from";
 const char kForwardDependentConfigsFrom_HelpShort[] =
     "forward_dependent_configs_from: [label list] Forward dependent's configs.";
 const char kForwardDependentConfigsFrom_Help[] =
     "forward_dependent_configs_from\n"
@@ skipping 104 matching lines @@
     INSERT_VARIABLE(ForwardDependentConfigsFrom)
     INSERT_VARIABLE(Ldflags)
     INSERT_VARIABLE(Sources)
   }
   return info_map;
 }
 
 #undef INSERT_VARIABLE
 
 }  // namespace variables